For live-bearing fish, the pregnancy length depends on the species of fish and varies considerably. For instance, according to the MCH Portal, platies have a gestation period of up to 7 weeks. On the other hand, mollies have pregnancies of about 4 weeks, according to Fish Lore.
Live-bearing fish are most frequently from the family Poeciliidae. Fish that give birth to live young include platies and mollies, as well as guppies, Endler's livebearers, swordtails and mosquito fish. These fish give birth to live, free-swimming young that can fend for themselves making them a better choice for novice fish raisers compared to egg-laying fish.